DIY strange thermometer

[fluidistic]

I am thinking to make a strange thermometer, based on the thermal expansion of a small aluminium-like rod/cylinder. I would attach one end of this aluminium rod to a vertical ceramic wall, and the other side would push against a piezoresistive material. I would then measure the resistance of this material. The resistance would indicate the pressure applied by the aluminium rod which in turn indicates how much the aluminium rod has expanded.
Which kind of piezoresistive material could I buy? I see several models on aliexpress and other websites, but I don't know if they are appropriate for such a use-case.

 

[anorlunda]

That sounds fun. I just tried this search. It resulted in several hits that may interest you.

https://duckduckgo.com/?q=thermoresistive&ia=web

 

[Borek]

Any tensometric beam would do, no? These are quite cheap.

 

[jrmichler]

I have done some experimenting with piezoresistive materials with mixed success.

One possibility is self limiting heat trace tape. The conductive polymer between the two wires is designed to vary resistance with temperature, but (if I recall correctly) is also sensitive to pressure.

Manganin strain gauges are designed to vary resistance with pressure. I used some of these to build a compact force transducer once, but did not take it to completion: https://micro-measurements.com/pca/special-use-gages/pressure_pulse. Put the strain gauge between two metal surfaces, and push. You will need a strain gauge signal conditioner to read it.

MTA: I once made a mixture of dry graphite and polyester resin. With enough graphite to make it as viscous as peanut butter, it became electrically conductive. It would not surprise me if the electrical resistance of the hardened mixture had a pressure sensitivity. Worth a try.

 

[Tom.G]

A Maganin pressure gauge is probably not the ideal choice due to low sensitivity, about 1/4% resistance change per kilobar (14 500psi).

See: https://strainblog.micro-measurements.com/content/manganin-pressure-gages

 

[jrmichler]

That is easily readable using normal strain gauge signal conditioners. Those have accuracy and repeatability of about one microstrain, which is about 2E-6 ohms / ohm with a typical gauge factor of two. Typical strain gauge signal conditioners do not have bandwidth capable of measuring blast forces, so other means of measuring fast changes in resistance are needed.

The next calculation is to find the stress in an aluminum bar clamped inside a material with a low TCE, then translate that to pressure on the gauge. Keeping in mind that the gauge has finite thickness and is encapsulated in plastic.

A fun little project.